On a simple H&R block Calculator a family of four earning $50,000 annually with no special deductions owes exactly $0 in federal income tax.
Calculator
The same family earning $72,000 and deducting RE taxes and mortgage interest owed $1,548.
That same family bumped up to $105,000 owe $7,388 in federal income tax.
I'm self employed and so is my wife. Many small businesses may not offer medical insurance but most have access to plans that aren't paid for by the company. It is more expensive (I currently pay about $9,000 in medical insurance premiums) but that is a risk/reward calculation for the individuals.
I'll stick with my earlier statement - if they earn so little as to be unable to pay for insurance they most likely earn so little they aren't paying income taxes either.